This invention relates to an apparatus for measuring photoelasticity and in particular to an optical system employing circular polarization for light of a wide wavelength region including the infrared region.
An apparatus disclosed in a Japanese reference "Oryoku Sokutei Ho (Stress Measuring Methods)" pp. 473-692, particularly pp. 492-503, published by Asakura Publishing Co., November 1964 has been known as an apparatus for measuring photoelasticity. However, the objects to be measured by means thereof are restricted to glass and plastic, which are transparent for the visible light region and white light (wavelength 400-760 nm) emitted by a tungsten lamp, D line (wavelength 589 nm) emitted by a natrium lamp, and light (wavelength 400-580 nm) emitted by a mercury lamp, that have been used as a light source therefor. In order to obtain the circular polarization necessary for the measurement of the photoelasticity, a quarter wavelength plate is used. Among the light sources described above, for the white light and the ultraviolet light, quarter wavelength plates for their center wavelength 580 nm and 490 nm, respectively, are used. Further, in the case where the light emitted by the light source is monochromatic as D line emitted by a natrium lamp, a phase shifter made of mica and corresponding to a wavelength of 147.25 nm is used as a quarter wavelength plate.
For this reason, when photoelasticity is measured by using a light source emitting light of wide wavelength region, it is difficult to obtain correctly circularly polarized light and measurement precision is not taken sufficiently into consideration. Further, in the case where a plurality of kinds of monochromatic light sources are used, they give rise to a problem similar to that of the light source stated above. As the result, an operation to interchange quarter wavelength plate in the course of a measurement is necessary, which complicates the measurement.
A stress measuring method, using the infrared photoelastic effect is disclosed in, for example, JP-A-No. 57-191504.
On the other hand, a catalog of an apparatus for measuring photoelasticity in the infrared region (Photolastic Inc. Model 501, Infrared polariscope) can be cited as a recent publication. However, this apparatus works only for a wavelength of 1.13 .mu.m and doesn't meet the needs to measure photoelasticity by using a light source emitting light of wide wavelength region.